Problems arise commonly as a result of pigment agglomeration or sediment formation when pigments are incorporated in water dilutable resins, particularly for water based coatings. To produce stable, water dilutable lacquers it is necessary thoroughly to wet the pigments and to grind them in appropriate dispersing apparatus to a very fine particle size such as smaller than 10.mu., and suitably smaller than 3.mu.. If low solvent containing, water dilutable dispersions or emulsions are needed then the shearing forces that arise are inadequate due to the very low viscosities. Therefore, in these cases it is necessary to substitute for a part of the binder that is used a paste forming resin in which the pigments are formed into a paste. These so called pigment pastes or grinding pastes are then mixed with the remaining binder to be lacquerified or otherwise completed. Once the object has been sprayed, a transparent one- or two-component lacquer is generally applied after drying, or on exposure to air, whereupon the two layers are jointly baked.
The paste forming resins to be used for such grinding pastes, also referred to as "pasting" resins, binders or pigment bearing resins, have a variety of required characteristics. The paste binder should, for instance:
have good compatibility with the principal binder in the film and, at the required rate of addition, cause no impairment of the lacquer characteristics; PA1 be capable adequately of wetting the pigments, to enable even fine dispersion with a minimum of energy; PA1 possess a viscosity within a range enabling paste formation with as high a pigment concentration as possible; PA1 prevent pigment flocculation during lacquerification; PA1 have a good shelf life for long periods of time, i.e. the pastes must not thicken, nor may the pigments form sediments or agglomerates; PA1 contain only such solvents as do not cause the particles of the main binder to swell; and PA1 cause no loss or other changes in color.
Water dilutable pigmented lacquers are used as primers, fillers, unit components of metallic base coats, e.g. with the aid of high molecular weight polyacrylate emulsions, polyurethane dispersions, polyester, or alkyd dispersions. Special combinations for the production of metallic base coats, which are subsequently sprayed conventionally dissolved, or in transparent aqueous lacquers have been described in European patents Nos. 260,447, and 297,576. The paste forming resins described in these publications do not, however, allow the basic characteristics of the lacquers present in the total system to become fully apparent. Polyester and acrylate resins are mentioned, for instance, in European patent No. 260,447. Although the suitable acrylate resins possess very good wetting characteristics, they are not fully compatible with the polyurethane dispersions or the polyesters grafted with acrylic monomers. This results in an impairment of the metallic effect.
In European patent No. 297,576, on page 10, lines 38 to 49, urethane modified polyesters with an acid number from 40 to 100 are described in general terms. These polyesters are produced by the reaction of a saturated polyester containing OH groups, with one or more aromatic, cycloaliphatic, and/or aliphatic polyisocyanates, at an equivalent OH group: isocyanate group ratio from 10:1 to 1:1. Examples of this type of paste forming resin are not given in that publication. Practical tests prove that due to their high acid number, these binders impair the moisture resistance values of the latter:
German patent publication No. 3,409,080 describes metal effect lacquers in accordance with the two layers wet-in-wet lacquering process with emulsifier containing aqueous dispersions based on oil free polyesters, alkyd resins, and/or acrylate resins with an acid number not higher than 30 and an OH number not higher than 150, in combination with water soluble amino resins. The examples refer only to a urethanized alkyd resin modified with fatty acid. There is no mention of any suitability as paste forming resins.